Diagnostics in tmr sensors

ABSTRACT

A computer-implemented method includes, by one or more processors in electronic communication with a tunneling magnetoresistive sensor, wherein the tunneling magnetoresistive sensor is a component of a magnetic storage drive configured to read magnetic data from a magnetic storage medium, detecting a short across the tunneling magnetoresistive sensor, measuring a change in resistance of the tunneling magnetoresistive sensor, measuring a change in voltage amplitude for the tunneling magnetoresistive sensor, and dividing said change in voltage amplitude by said change in resistance to yield a ratio. The computer-implemented method further includes, responsive to the ratio being greater than a predetermined ratio threshold, determining that the short is caused by a magnetic shunt. A corresponding computer program product and computer system are also disclosed.

BACKGROUND

The present invention relates generally to the field of magnetic tapereaders, and more particularly to diagnosing and recovering fromhardware failures in tunneling magnetoresistive sensors.

Tunneling magnetoresistive (“TMR”) sensors are microelectronic devicesthat are characterized by a change in electrical resistance in thepresence or absence of a magnetic field. Magnetic storage devices, suchas magnetic tape drives and hard disk drives, rely upon TMR sensors toread data from magnetic media. Different regions of magnetic mediacorrespond to bits of data, each of which can apply either of twodifferent magnetic field states to the TMR sensor. The value of eachbit, one or zero, can be determined electronically by measuringresistance across the TMR sensor such that one state may becharacterized by a relatively high resistance and the othercharacterized by a relatively low resistance.

As with all microelectronic devices, TMR sensors experience hardwarefailures of various kinds. In particular, TMR sensors are often locatedat an air bearing surface, which exposes them to various kinds ofexternal damage. Using software, for example controller firmware ordriver software that operates a tape drive or magnetic disk drive,engineers can diagnose and mitigate various failures, thereby allowingdevices to continue to function despite some operational defect at themicroelectronic device level. Also with the aid of software, engineerscan diagnose failures in devices that have been rendered inoperable orsubjected to failure analysis.

SUMMARY

A computer-implemented method includes, by one or more processors inelectronic communication with a tunneling magnetoresistive sensor,wherein the tunneling magnetoresistive sensor is a component of amagnetic storage drive configured to read magnetic data from a magneticstorage medium, detecting a short across the tunneling magnetoresistivesensor, measuring a change in resistance of the tunnelingmagnetoresistive sensor, measuring a change in voltage amplitude for thetunneling magnetoresistive sensor, and dividing said change in voltageamplitude by said change in resistance to yield a ratio. Thecomputer-implemented method further includes, responsive to the ratiobeing greater than a predetermined ratio threshold, determining that theshort is caused by a magnetic shunt. A corresponding computer programproduct and computer system are also disclosed.

In an aspect, the computer-implemented method further includes,responsive to the ratio being less than the predetermined ratiothreshold, determining that the short is caused by at least one of adielectric breakdown and a nonmagnetic shunt.

In an aspect, the computer-implemented method further includes,responsive to determining that the short is caused by the magneticshunt, setting a bias voltage across the tunneling magnetoresistivesensor to a normal value, wherein the normal value is effective in theabsence of the short.

In an aspect, the computer-implemented method further includes,responsive to determining that the short is caused by the dielectricbreakdown, limiting a bias voltage across the tunneling magnetoresistivesensor to no greater than a voltage limit, wherein the voltage limit isa value effective for protecting the dielectric breakdown from growing.

In an aspect, another computer-implemented method includes, by one ormore processors in electronic communication with a tunnelingmagnetoresistive sensor, responsive to detecting an operational anomalyin the tunneling magnetoresistive sensor, measuring a first resistancechange in the presence of a positive bias current, measuring a secondresistance change in the presence of a negative bias current. Thecomputer-implemented method further includes. responsive to at least oneof the first resistance change and the second resistance change beingmore positive than expected based on a device geometry for the tunnelingmagnetoresistive sensor, returning a probable determination that theoperational anomaly is caused by a short across the tunnelingmagnetoresistive sensor. A corresponding computer program product andcomputer system are also disclosed.

In an aspect, another computer-implemented method includes, by one ormore processors in electronic communication with a tunnelingmagnetoresistive sensor, wherein the tunneling magnetoresistive sensoris a component of a magnetic tape drive configured to read magnetic datafrom a magnetic tape, detecting a short across the tunnelingmagnetoresistive sensor contemporaneously with the tape running acrossthe tunneling magnetoresistive sensor, and measuring a voltage amplitudeacross the tunneling magnetoresistive sensor as a function of afractional current through the tunneling magnetoresistive sensor toyield a voltage amplitude data set. The computer-implemented methodfurther includes, responsive to the voltage amplitude data set fitting apower law, wherein the power law comprises an exponent, and wherein theexponent is greater than an exponent threshold, determining that theshort is caused by a magnetic shunt. A corresponding computer programproduct and computer system are also disclosed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a functional block diagram illustrating an operationalenvironment for a TMR diagnostic program, in accordance with at leastone embodiment of the invention.

FIG. 2 is a flowchart depicting operational steps for a TMR diagnosticprogram, in accordance with at least one embodiment of the invention.

FIG. 3 is a flowchart diagram depicting alternative operational stepsfor a TMR diagnostic program, in accordance with at least one embodimentof the present invention.

FIG. 4 is a flowchart diagram depicting alternative operational stepsfor a TMR diagnostic program, in accordance with at least one embodimentof the present invention.

FIG. 5A is a plot of resistance versus time in an experimentdemonstrating resistance drop in a TMR sensor that has experienceddielectric breakdown as compared with several that have not.

FIG. 5B is a plot of change in voltage amplitude versus change inresistance for a TMR sensor that has experienced dielectric breakdownthrough a pin-hole.

FIG. 6A is a plot of the change in voltage amplitude at a constantcurrent versus change in resistance for TMR sensors with shorts due todielectric breakdown, wherein a first group of TMR sensors experienceddielectric breakdown from Electrical Over Stress (EOS) pulses.

FIG. 6B is a plot of the change in voltage amplitude at a constantcurrent versus change in resistance for TMR sensors with shorts due todielectric breakdown, wherein a second group of TMR sensors experienceddielectric breakdown from EOS pulses.

FIG. 7 is a plot of voltage amplitude versus fractional current for TMRsensors that have experienced dielectric breakdown from EOS pulses.

FIG. 8A is an atomic force microscopy image at 25 μm of a tape readerthat has experienced a lapping scratch.

FIG. 8B is an atomic force microscopy image at 5 μm of a tape readerthat has experienced a lapping scratch.

FIG. 8C is a magnetic force microscopy image at 5 μm of a tape readerthat has experienced a lapping scratch.

FIG. 9A is a plot of resistance versus time for a group of TMR sensors,one of which has experienced a lapping scratch.

FIG. 9B is a plot of voltage amplitude versus time for a group of TMRsensors, one of which has experienced a lapping scratch.

FIG. 10A is a plot of change in voltage amplitude at constant currentversus change in resistance due to a short occurring while runningagainst tape for −40%<ΔR<0.

FIG. 10B is a plot of change in voltage amplitude at constant currentversus change in resistance due to a short occurring while runningagainst tape for −60%<ΔR<0.

FIG. 11A is a plot of fractional current in a first TMR sensor when ashort occurs while tape is running.

FIG. 11B is a plot of fractional current in a second TMR sensor when ashort occurs while tape is running.

FIG. 12A is a plot of resistance versus bias current for a TMR sensorprior to a short.

FIG. 12B is a plot of resistance versus bias current for a TMR sensorafter a short.

FIG. 13 is a plot of change in amplitude versus change in resistanceacross the TMR sensor for a first group with shorts due to lappingscratches and a second group with shorts due to dielectric breakdown.

FIG. 14A is plot of a distribution of ΔAmp/ΔR from shorts due todielectric breakdown with ΔR≤50%.

FIG. 14B is plot of a distribution of ΔAmp/ΔR from shorts due todielectric breakdown with ΔR≤50% and from lapping scratches with ΔR≤20%.

FIG. 14C is plot of a distribution of ΔAmp/ΔR from shorts due todielectric breakdown with ΔR≤50% and from lapping scratches with ΔR≤65%.

FIG. 15A is a side profile view schematic depiction of a TMR sensor asconsidered in magnetic modeling with respect to at least one embodimentof the invention.

FIG. 15B is a plan view schematic depiction of a TMR sensor asconsidered in magnetic modeling, with respect to at least one embodimentof the present invention.

FIG. 16A is a schematic depiction of a model for magnetic shielding of ashunt.

FIG. 16B is a theoretical plot of magnetic field strength versosmaterial depth for shielded and normal shunts.

FIG. 17A is a plot of the drop in magnetic field inside of a TMR sensorwith width of magnetic shunt having a thickness of 1 nm, 2 nm, 3 nm, 6nm and 10 nm.

FIG. 17B is a plot of the drop in magnetic field inside of a TMR sensorversus permeability of the magnetic shunt for shunt widths of 200 nm and500 nm.

FIG. 17C is a plot of the drop in magnetic field inside of a TMR sensorversus the depth of the magnetic shunt for shunt widths of 200 nm and500 nm.

FIG. 18 is a block diagram depicting various logical elements for acomputer system capable of executing program instructions, in accordancewith at least one embodiment of the present invention.

DETAILED DESCRIPTION

Referring now to the invention in more detail, FIG. 1 depicts anoperational environment for a TMR diagnostic program 101, in accordancewith at least one embodiment of the present invention. Broadly, the TMRdiagnostic program 101 may be understood as a method to recover driveperformance following shoring of a TMR sensor by distinguishing theshorting mechanism and setting the bias voltage accordingly. In thedepicted embodiment, a magnetic storage control computer 100 is embeddedin a magnetic tape drive. In alternative embodiments the magneticstorage control computer 100 may be embedded in a magnetic hard orfloppy disk drive or any other magnetic storage medium for which TMRsensors may be applied as a reading device. In the depicted embodiment,the magnetic storage medium 115 (i.e., a tape, disk, or other magneticdata storage material) is proximate to a TMR sensor 109. In general, theTMR (a tunneling magnetoresistive sensor) may be understood as acomponent of a magnetic storage drive configured to read magnetic datafrom a magnetic storage medium.

More particularly, with respect to various embodiments, a TMR sensor isa multi-layer device which include three significant layers, amongothers: a pinned layer, a tunnel junction, and a free layer. Theresistance of the TMR depends on the alignment of the magnetization ofthe pinned layer versus the free layer. When the magnetization of thepinned layer and free layer are aligned, the resistance is lowest.Correspondingly, when the pinned layer and free layer are anti-aligned,the resistance is highest. To fabricate a TMR with a reasonableresistance, the tunnel junction is very thin. For read heads configuredto read from tape or hard disks, the tunnel junction thickness may be ˜1nm or lower. Because of the thin tunnel junction layer, shorts acrossthe tunnel junction can occur, wherein current is shunted through theshort rather than through the TMR layer.

The inventors have observed and/or recognized three different shortingmechanisms: (1) dielectric breakdown, wherein the dielectric material ofthe TMR sensor collapses in a narrow region (i.e., a pin-hole), as withan electric arc through air; (2) lapping scratches, wherein the leadmaterial is bridged across the TMR sensor, and (3) tape scratches, whichoccur when running tape, and wherein the tape carries some form ofparticulate material across the TMR sensor, which results in a scratchacross the read element, and, potentially, a conductive short across thethin tunneling barrier of the TMR sensor, in view of the high contactpressure of the tape across tape bearing surface. The inventors haveobserved and/or recognized that, in all three cases, if the voltageacross the tunnel junction is maintained constant after the short hasoccurred, then the voltage amplitude of the TMR sensor in response froma magnetic field from the tape medial should revert to its value priorto the short. It should be noted that voltage amplitude may beunderstood as the peak-to-peak change in the resistance of the TMRsensor multiplied by the applied current to the TMR sensor. Thus, in thecase of a conductive short in parallel with a TMR sensor, the voltageamplitude will be reduced, as described below. Thus, the inventors haveobserved and/or recognized, the TMR can still function after the short.However, the inventors have observed and/or recognized that a problemwith the short from dielectric breakdown is that high levels of currentpassing through the small shorting pillar will likely cause the size ofthe pillar to grow, which in turn causes the shorting resistance todecrease without bound until the drive can no longer supply sufficientcurrent to operate. The inventors have further observed and/orrecognized that his problem is not present in shorts that occur due tolapping scratches or tape scratches.

Thus, the inventors have further observed and/or recognized that a meansof distinguishing between the different sources of shorting can beemployed to enable the continued use of TMR sensors that have shorted.The inventors have further observed and/or recognized that thepreviously known means of distinguishing between shorts due todielectric breakdown and shorts due to lapping or tape scratches includeonly failure analysis. In failure analysis, a scratch is visible in anatomic force microscopy image of the air bearing surface in which theTMR sensor is embedded (e.g., FIGS. 8A-8B), similarly, under atomicforce microscopy imaging, a TMR having experienced dielectric breakdownwill not have a surface scratch. The inventors have further observedand/or recognized that there dielectric breakdown can occur at the tapebearing surface of the TMR, however such an event would be visible as adot in an atomic force microscopy or scanning electron microscopy image,and the imaged shape would be visibly and diagnostically distinct fromthat of a scratch. In any event, failure analysis of TMR sensorsrequires the TMR sensor that has experienced a short to be taken out ofservice and physically brought to a laboratory setting where it can beexamined with the aforementioned microscopy technologies and otherimaging techniques. Thus, the inventors have further observed and/orrecognized, a diagnostic method based solely on in situ electricalmeasurements of the TMR sensor would permit control software to keep theTMR sensor in service after detecting a short. It will be understoodthat the aforementioned problems solved by some embodiments of theinvention and/or advantages over the prior art exhibited by someembodiments of the invention are not intended as limitations on theinvention as claimed, and any particular advantage need not necessarilybe present in all embodiments.

Referring still to FIG. 1, in the depicted embodiment, the magneticstorage medium 115 produces a magnetic field 113 (designated by themagnetic field strength symbol {right arrow over (H)}). The magneticfield 113 changes in direction and/or magnitude with the data encodedupon that region of the magnetic storage medium 109 that is proximate tothe TMR sensor 109. As the magnetic field 113 changes, the magnetizationof the free layer in the TMR sensor 109 rotates, and the resistance ofthe TMR sensor 109 correspondingly changes. The TMR sensor 109 may beunderstood as a resistor 109A, the resistance of which may be measuredvia a digital measurement unit 104. The change in the TMR resistancewith applied field times the bias current through the TMR sensor gives avoltage amplitude. It will be understood that FIG. 1 is a schematicrepresentation only and that it is not the intent of the Applicant tosuggest that the TMR sensor 109 literally contains a resistor 109A, butrather that the TMR sensor 109 has the property of a measurableresistance represented by a notional resistor 109A. The digitalmeasurement 104 unit may be understood to include any circuit elements,sensors, logic gates, firmware, etc. that enable electrical measurementsof the TMR sensor 109.

Referring still to the embodiment of FIG. 1, in addition to resistance,the digital measurement unit 104 may apply a bias current to the TMRsensor 109. The bias current may be in either direction across the TMRsensor 109 or it may alternate. The digital measurement unit 104 maymeasure the output voltage or voltage amplitude as a result of applyinga defined bias current, which can include, selectively, direct currentin either direction and alternating current over a range of frequencies.Similarly, the digital measurement unit 104 may measure resistancesimultaneously with the application of a bias current so as to measureresistance as a function of changes in the bias current. Further, thedigital measurement unit 104 may measure resistance repetitiously orcontinuously such that various effects can be observed as a function ofchanges in resistance.

Referring still to the embodiment of FIG. 1, the magnetic storagecontrol computer 104 is in electronic digital communication with thedigital measurement unit 104. For example, the digital measurement unit104 may include one or more analog-digital converter circuits wherebythe electrical properties of the TMR sensor 109 are made accessible asdigital values to software operating on the magnetic storage controlcomputer 100, such as the TMR diagnostic program 101. The TMR diagnosticprogram 101 may be understood as performing various electricalmeasurements of the TMR sensor 109 by accessing the digital measurementunit.

More generally, the electrical measurements of the TMR sensor 109 may beunderstood in terms of several relationships, which the inventors haveidentified. For a tunnel junction with a resistance R_(mro) and aparallel short with a resistance R_(s), current is diverted throughR_(s), and overall resistance R for the tunnel junction is:

$\begin{matrix}{R = {R_{mro} \cdot \frac{R_{s}}{\left( {R_{s} + R_{mro}} \right)}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

The change in resistance ΔR_(TMR) for the same tunnel junction is:

$\begin{matrix}{{\Delta \; R_{TMR}} = {{100{\% \cdot \frac{\left( {R - R_{mro}} \right)}{R_{mro}}}} = {{- 100}{\% \cdot \frac{R_{mro}}{\left( {R_{s} + R_{mro}} \right)}}}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

For a constant current of I_(mro) applied to the TMR sensor, the currentthrough the tunnel junction, I_(mr), is decreased by the ratio,

$\frac{R}{R_{mro}}\text{:}$

$\begin{matrix}{I_{mr} = {\frac{V}{R_{mro}} = {{I_{mro} \cdot \frac{R}{R_{mro}}} = {I_{mro} \cdot \frac{R_{s}}{\left( {R_{s} + R_{mro}} \right)}}}}} & {{Equation}\mspace{14mu} 3}\end{matrix}$

When a magnetic field of strength H_(f) is applied to a TMR sensor, thechange in resistance, ΔR_(TMRH), is proportional to H_(f) and is afraction F_(TMR) of the resistance of the TMR sensor:

ΔR _(TMRH) =F _(TMR) ·H _(f) ·R _(mro)   Equation 4

The voltage amplitude Amp is the current times the resistance change,Amp_(o) is the voltage amplitude at a current of I_(mro) prior to theshort:

$\begin{matrix}{{Amp} = {{I_{mr} \cdot F_{TMR} \cdot H_{f} \cdot R_{mro}} = {{I_{mro} \cdot F_{TMR} \cdot H_{f} \cdot R_{mro} \cdot \frac{R}{R_{mro}}} = {{Amp}_{o} \cdot \frac{R}{R_{mro}}}}}} & {{Equation}\mspace{14mu} 5}\end{matrix}$

Thus, under constant current, the change in voltage amplitude ΔAmp is:

$\begin{matrix}{{\Delta \; {Amp}} = {{100\% \frac{\left( {{Amp} - {Amp}_{o}} \right)}{{Amp}_{o}}} = {\Delta \; R}}} & {{Equation}\mspace{14mu} 6}\end{matrix}$

Thus, the change in amplitude, expressed as a percentage (or, as aratio, if the factor of 100% is removed), if equal to the change inresistance, also expressed as a percentage (or, as a ratio, if thefactor of 100% is removed), and thus:

$\begin{matrix}{\frac{\Delta \; {Amp}}{\Delta \; R} = 1} & {{Equation}\mspace{14mu} 7}\end{matrix}$

Equation 7 provides a theoretical prediction for the ratio of the changein voltage amplitude to the change in resistance, however the inventorshave observed, by experiment, that the theoretical value is not realizedin practice. FIG. 5A is a plot of TMR resistance versus time for fiveTMR sensors, one of which has suffered dielectric breakdown during aconstant voltage stress experiment. FIG. 5A shows how the shortcontinues to grow for the TMR sensor which has experienced dielectricbreakdown, as evidenced by the change in resistance continuing to dropover time. The inventors have further observed and/or recognized that,if the voltage stress on the TMR sensor with a dielectric breakdown isdecreased sufficiently, then the short will stop growing and theamplitude will remain stable.

FIG. 5B is a plot of ΔAmp versus ΔR_(TMR) with the predicted line ofEquation 7. The cluster of low values on the left shows how, afterdielectric breakdown occurs, if the voltage remains high across theshorted TMR sensor, then the size of the shorting pillar grows and theamplitude continues to drop. This leads to failure of the TMR sensor. InFIG. 5B, the dotted line shows the ideal Equation 7, with a slope of 1,and is the value determined when fitting the data when forcing

$\frac{\Delta \; R}{\Delta \; {Amp}} = 0$

at R=0. The lighter line shows a linear regression of the data pointswith a slope of 1.56.

FIG. 6A is a plot of the change in voltage amplitude at a constantcurrent versus change in resistance for TMR sensors with shorts due todielectric breakdown for group of 20 TMR sensors that experienceddielectric breakdown from EOS pulses over a short time (˜25 ns-100 ns).The slope (i.e., average) of ΔAmp/ΔR_(TMR) was 1.27±0.05. FIG. 6B is asimilar plot for three TMRs that experienced dielectric breakdown fromEOS over a long time (˜10 s-days). The slope of ΔAmp/ΔR_(TMR) was1.38±0.08. These results are 27% and 38% higher, for FIGS. 6A and 6B,respectively, than for the ideal case of Equation 7. The inventorshypothesize that the higher slopes are explained by and/or due to theloss in area in the TMR due to the short.

FIG. 7 is a plot of amplitude versus fractional current through the sameTMR sensors whose measurements are presented in FIG. 6B. For the idealcase of a shorting resistor in parallel with the TMR sensor, the slopeof Amplitude (Amp) versus current through the TMR (ITMR) is expected tobe linear with a slope of unity (as in Equation 7). Data for all threeshorted TMR sensors was fit with a linear equation with a slope of1.38±0.09%/% and a zero intercept of −0.28±0.09%/%, as shown.

In all of the cases studied by the inventors, the dielectric breakdownshorts generated by EOS pulses resulted in amplitude drops which aresimilar to, but higher than expected from an ideal parallel short model.The amplitude versus the effective current through the shorted TMRsensor can be fit with a linear equation with a slope of between about 1to 1.6%/%. The change in amplitude versus change in resistance can befit with a slope of 1.3%/% within a range of ±0.3%/%.

In addition to dielectric breakdown, lapping and tape scratches are alsoa cause of shorts in TMR sensors. FIG. 8A is an atomic force microscopyimage at 25 μm of a tape reader that has experienced periodic scratchesfrom a particle on the tape. The spacing between the scratches is thephysical distance the tape is stepped over between wraps of the tape(also known in the art as “track pitch”). FIG. 8B is an atomic forcemicroscopy image at 5 μm of the same tape reader. FIG. 8C is a magneticforce microscopy image at 5 μm of the same tape reader. In theexperiment shown, the scratches were 15.8 nm deep, 2.02 μm spacingbetween, and 0.57 μm wide. As shown, the scratch features demonstratehow a particulate embedded in the tape can be dragged across the TMRsensor at the air bearing surface, causing a short.

FIG. 9A is a plot of resistance versus time for a group of TMR sensors,one of which suffered a short due to a particulate on the tapescratching the surface of the TMR sensor and dragging metal materialacross the tunnel junction, causing a short in parallel with the bulk ofthe TMR. FIG. 9B is a plot of the 2T-Amplitude from a read-back signalfrom magnetic transitions written on the tape. 2T is a fundamentalperiod of data density for a given tape. The resistance dropped by −15%while the Amplitude dropped by 43%, or a ratio of

$\frac{\Delta \; {Amp}}{\Delta \; R_{TMR}}$

of 2.9. Two important observations are: (1) The ratio of

$\frac{\Delta \; {Amp}}{\Delta \; R_{TMR}}$

of 2.9 is almost 3 times the expected value for a simple parallel short,and (2) the resistance did not continue to drop in this case, so thevoltage can be increased to yield higher output.

FIGS. 10A and 10B are a plots of the change in voltage amplitude at aconstant current versus change in resistance for TMR sensors with shortsdue to scratches across the tunnel junction from tape wear. For FIG.10A, the relevant range is −40%<ΔR≤0. For FIG. 10B, the relevant rangeis −60%<ΔR≤0%. The observed change in resistance due to the short wasbetween −10% and −25%. The inventors concluded that the reason for therange in ΔR_(TMR) is because the TMR studied suffered multiplescratches, with the resistance dropping with each additional scratch.The inventors observe that the slope,

$\frac{\Delta \; {Amp}}{\Delta \; R_{TMR}},$

is 1.74 for these parts, is significantly higher than the predictedvalue of 1 for a simple parallel short, and larger than the valuesmeasured from dielectric breakdown. The inventors further observe thatthe results are essentially identical for 2T and 8T amplitudes, and thusthey conclude that the additional amplitude drop is not due to Wallacespacing losses. Further, for ΔR>−40% (i.e., for Amplitude losses of lessthan 40%), the drop in voltage amplitude with change in resistance ishigher than calculated from a simple parallel resistance shunt, and forΔR≤−40%, (i.e., for Amplitude losses of greater than 40%), the slope in

$\frac{\Delta \; {Amp}}{\Delta \; R}$

decreases.

FIGS. 11A and 11B are plots of normalized 2T and 8T amplitudes versusfractional current through the shorted TMR sensor, where the shorts aredue to scratches on tape, as in FIG. 10. The data is fit to a power law,with an exponent β of 2.0 and 2.3:

$\begin{matrix}{I_{TMR} = {I_{0}\frac{R}{R_{{TMR}_{0}}}}} & {{Equation}\mspace{14mu} 8} \\{{Amp}_{Norm} = {\left( \frac{I_{TMR}}{I_{0}} \right)^{\beta} = I_{{TMR}_{Norm}}^{\beta}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

In Equations 8 and 9 above, R is the resistance with the short, R_(TMR)is the initial resistance of the TMR sensor, Amp_(Norm) is the amplitudenormalized to the TMR's pre-short value, I₀ is the bias current appliedto the sensor in measuring the amplitude, and ITMR is the currentflowing through the TMR sensor in the presence of the short. ITMR_(Norm)is the current through the TMR sensor normalized to I₀. In the idealcase, β is 1.0.

The slope of Amp_(Norm) versus ITMR_(Norm) varies with ITMR_(Norm):

$\begin{matrix}{\frac{\Delta \; {Amp}_{Norm}}{\Delta \; {TMR}_{Norm}} = {\beta \cdot I^{\beta - 1}}} & {{Equation}\mspace{14mu} 10}\end{matrix}$

The slope is maximum at ITMR_(Norm)=1, and equal to 1. For the partsstudied by the inventors, the slope is between 2 and 3 for ITMR_(Norm)greater than about 0.8, whereas the slope is 1 for the ideal model of ashorting resistor.

FIG. 12A is a plot of resistance versus bias current for a TMR sensorprior to a short. As shown, the resistance decreases with bias currentfor both positive and negative polarity, which is the expected behaviorfor a TMR device, and TMR sensors without a short have been observed todecrease resistance with increasing voltage across the TMR sensor.Resistance is therefore expected to decrease monotonically for at leastone polarity. FIG. 12B shows resistance increasing after a short forboth polarities. The increase in resistance is understood by theinventors to be due to joule heating because of the principle that theresistance of a metal increases with its temperature. Therefore, theincrease in resistance indicates that a thin short brides across thetunnel junction and is heating up due to the large current densitythrough the short. While the resistance of a tunnel junction willdecrease with increasing voltage across it, the resistance of a metalwill increase. The inventors have observed and/or recognized that thistechnique of determining a short may be applicable if the history of agiven TMR is not known (e.g., if it is installed in a newly manufacturedand unused read head), and therefore it is not possible to take adifference between a current and a previously measured resistance value.The inventors have further observed and/or recognized that theaforementioned method may also be applicable in distinguishing between adrop in resistance due to a magnetically induced change and a physicalshort. If a short is detected by a drop in measured resistance, but thechange in resistance versus bias current is normal, then it is likely amagnetically induced change in the sensor and not physically inducedshort such as a scratch or dielectric breakdown.

FIG. 13 compares the properties of shorts from drive scratching withthose from dielectric breakdown. FIG. 13 is a plot of change in voltageamplitude versus change in TMR resistance for a first group of TMRsensors with shorts that occurred in a drive due to scratches and asecond group of TMR sensors that intentionally experienced dielectricbreakdown. As shown, dielectric breakdowns, denoted by the whitesquares, are shown to have a linear profile over a range of tested ΔR,while the scratches, denoted as black dots, have a nonlinear profile inaccordance with the above-described equations. The white squares areshort from the drive of FIGS. 9A and 9B. Also shown is a fit to shortsdue to scratches using the concept of shielding at the ABS due tomagnetic material in the shunt, as discussed below. The dotted line fitsthe change in amplitude versus resistance for a resistive shunt combinedwith a magnetic shunt.

FIGS. 14A, 14B, and 14C demonstrate the diagnostic value of the

$\frac{\Delta \; {Amp}}{\Delta \; R}$

ratio. FIG. 14A shows the distribution of

$\frac{\Delta \; {Amp}}{\Delta \; R}$

from shorts due to dielectric breakdown taken from FIG. 13. FIG. 14Bintroduces

$\frac{\Delta \; {Amp}}{\Delta \; R}$

for scratches, which is generally and diagnostically greater than fordielectric breakdown, where the data is from FIG. 13 for parts withΔR≤10%. FIG. 14C shows the distribution of

$\frac{\Delta \; {Amp}}{\Delta \; R}$

from shorts due to dielectric breakdown and from Scratches withΔAmp≤65%, where the data is again taken from the data in FIG. 13. 93% ofthe parts with dielectric breakdown have a

${\frac{\Delta \; {Amp}}{\Delta \; R} < 1.7},$

and 100% of the scratches had a

$\frac{\Delta \; {Amp}}{\Delta \; R} > {1.7.}$

Thus the value of

$\frac{\Delta \; {Amp}}{\Delta \; R}$

is diagnostic for determining whether a short is from a scratch ordielectric breakdown.

The inventors have further studied a magnetic model related to thepresent invention. FIGS. 15A and 15B show the geometry of the modeledtape reader 1500. FIG. 15A is a side profile view of the modeled TMRsensor. FIG. 15B is a plan view of the same device. Conductive shields1501 and 1502 are opposed across a gap, which is bridged by a shunt1506. This attempts to model a tape scratch and/or lapping scratchwherein conductive material is dragged across the reader layers. Thefree layer 1504 sits in between the shields and provides its propertiesinto the model. FIG. 16A models the shields 1501 and 1502 as R₀ andR_(s). FIG. 16B describes the expected H field behavior for a shieldedand unshielded reader. The shield effects in the depicted model are asfollows:

$\begin{matrix}{\mspace{79mu} {R = \frac{R_{0} \cdot R_{s}}{\left( {R_{s} + R_{0}} \right)}}} & {{Equation}\mspace{14mu} 11} \\{\mspace{79mu} {{\Delta \; R} = {\frac{\left( {R - R_{0}} \right)}{R_{0}} = {- \frac{R_{0}}{R_{s} + R_{0}}}}}} & {{Equation}\mspace{14mu} 12} \\{ {I_{mr} = {\frac{V}{R_{0}} = {I_{b\; 0} \cdot \frac{R}{R_{0}}}}}} & {{Equation}\mspace{14mu} 13} \\{ {{{Signal}\text{:}\mspace{14mu} \Delta \; R_{amp}} = {A_{s} \cdot R_{0} \cdot \frac{H_{F}}{2}}}} & {{Equation}\mspace{14mu} 14} \\{{Amp} = {{{I_{mr} \cdot \Delta}\; R_{amp}} = {{I_{mr} \cdot A_{s} \cdot R_{0}} = {{I_{b\; 0} \cdot \frac{H_{F}}{2} \cdot A_{s} \cdot R} = {\left( \frac{H_{F}}{H_{N}} \right) \cdot {Amp}_{0} \cdot \frac{R}{R_{0}}}}}}} & {{Equation}\mspace{14mu} 15} \\{\mspace{79mu} {{\Delta \; {Amp}} = {\frac{\left( {{Amp} - {Amp}_{0}} \right)}{{Amp}_{0}} = {{\left( \frac{H_{F}}{H_{N}} \right)\left( {1 + {\Delta \; R}} \right)} - 1}}}} & {{Equation}\mspace{14mu} 16}\end{matrix}$

In the case of a simple short (i.e., no magnetic shielding, H_(F)=H_(N),and no magnetic shielding occurs:

$\begin{matrix}{{{If}\mspace{14mu} H_{F}} = {{H_{N}\text{:}\mspace{11mu} \frac{\Delta \; {Amp}}{\Delta \; R}} = 1}} & {{Equation}\mspace{14mu} 17}\end{matrix}$

In the case where a magnetic shielding is present, H_(F)=H_(S)<H_(N),magnetic shielding occurs. Thus, the signal is decreased even further ascompared with a simple electrical short.

$\begin{matrix}{\mspace{79mu} {{{{If}\mspace{14mu} H_{F}} = {H_{S} < {H_{N}\text{:}}}}{{{\Delta \; {Amp}}} = {{\left( {1 - \left( \frac{H_{S}}{H_{N}} \right)} \right) + {\left( \frac{H_{S}}{H_{N}} \right) \cdot {{\Delta \; R}}}} = {\left( {1 - \frac{H_{S}}{H_{N}}} \right) + {\frac{H_{S}}{H_{N}} \cdot {{\Delta \; R}}}}}}}} & {{Equation}\mspace{14mu} 18}\end{matrix}$

The above analytical model includes a both an electrical short and adecrease in the magnetic field reaching the sensor due to a magneticshielding effect. This shows that, if the magnetic field is somehowdecreased due to magnetic shielding, then with shielding, the ratio

$\frac{\Delta \; {Amp}}{\Delta \; R}$

will be larger (as per Equation 18) than in the case of a simpleconductive shunt (as per Equation 17).

Micromagnetic calculations were then done using a finite element model(FEM) including a magnetic shunt across the shields 1501 to 1502.

FIG. 17A shows the drop in magnetic field inside a TMR sensor versus thewidth of the magnetic shunt for shunt thickness of 1 nm, 2 nm, 3 nm, 6nm, and 10 nm. It should be noted that the sensor is 1500 nm wide, whichis the same value used for the sensors for the experiments on scratchingand dielectric breakdown. The magnetic field in the sensor dropsessentially linearly with width of the magnetic shunt. FIG. 17B showsthe drop in magnetic field inside the TMR sensor versus the depth(thickness) of the magnetic shunt for shunt widths of 200 nm and 500 nm.Note that the magnetic field loss saturates at around a 3 nm thickmagnetic shunt. This model explains very well the drop in amplitudeversus shunt resistance shown in FIG. 13. FIG. 17C shows a plot of themagnetic field inside TMR sensor versus permeability of the magneticshunt for shunt widths of 200 nm and 500 nm.

More generally, a short due to a scratch from a particulate on the tapeis expected to result in a drop in amplitude and the ratio of

$\frac{\Delta \; {Amp}}{\Delta \; R}$

is significantly higher than the value of 1 expected for a simpleparallel short across the tunnel junction which shunts current away fromthe TMR sensor and through the short. By contrast, a short due to adielectric breakdown across the tunnel junction is expected to result ina drop in amplitude, wherein

$\frac{\Delta \; {Amp}}{\Delta \; R}$

is close to the value of 1 expected for a simple parallel short. For aTMR sensor in a tape drive, when

${\frac{\Delta \; {Amp}}{\Delta \; R} \geq {dAdr}_{Limit}},$

then the short is most likely cause by a scratch from a particle on thetape. When

${\frac{\Delta \; {Amp}}{\Delta \; R} < {dAdr}_{Limit}},$

then the short is most likely due to dielectric breakdown. A firstchoice for the limit, dAdR_(Limit), is 1.7. A second choice of isdAdR_(Limit) 2.0. For values of

$\frac{\Delta \; {Amp}}{\Delta \; R}$

below 1.7, a magnetic shunt is unlikely. For values of

$\frac{\Delta \; {Amp}}{\Delta \; R}$

between 1.7 and 2.0, dielectric breakdown is unlikely. In the case of ascratch, the voltage across the TMR sensor can be increased to thelimits for unshorted TMRs, while in the case of dielectric breakdown,the voltage limit should be set low enough to avoid growth of theshorting pillar.

The drop in amplitude resulting from shorts across the tunnel junctionat the air bearing surface causes by scratches at the air bearingsurface from running tape drops more rapidly than expected from aparallel short. For ΔR_(TMR) above about −20%, the measured slope ofΔAmp versus resistance were all greater than 1.7, and several werebetween 2 and 3. Also, a plot of amplitude versus fractional currentthrough the TMR sensor with a parallel short were fit with a power lawwith an exponent between 2 and 2.3 rather than a linear curve asexpected for the ideal case.

Referring now to the embodiment depicted in FIG. 2, FIG. 2 is aflowchart diagram for a TMR diagnostic program 101, in accordance withat least one embodiment of the present invention. In the depictedembodiment, at step 200, the TMR diagnostic program 101 detects a shortacross the TMR sensor 109. The TMR diagnostic program 101 may detect ashort by monitoring resistance across the TMR sensor 109 (measurement ofresistance may generally include a measurement with respect to the samedevice with a known resistance at an earlier time) and observing asudden drop in resistance, which indicates that a new electric pathway(i.e., a short) has been made in the partially dielectric material ofthe TMR sensor 109. At step 210, the TMR diagnostic program 101 measuresthe change in resistance of the TMR sensor 109. The change in resistanceis computed by taking the difference between the measured resistanceafter the short and the original resistance before the short. Normally,the change in resistance will have a negative sign, since the shuntnormally causes resistance to drop.

Referring still to the embodiment depicted in FIG. 2, at step 220, theTMR diagnostic program 101 measures the change voltage amplitude for theTMR sensor 109. The change in voltage amplitude is computed by takingthe difference between the measured voltage amplitude after the shortand the original voltage amplitude before the short. Normally, thechange in voltage amplitude will have a negative sign, as shown in FIG.13.

Referring still to the embodiment depicted in FIG. 2, at step 240, theTMR diagnostic program divides the change in voltage amplitude by thechange in resistance to yield a ratio. As in Equation 7, the theoreticalexpected value of the ratio is 1, however, as described above, theobserved ratio is greater than one, and the inventors have observedand/or recognized that the value of the ratio is, at least in part,diagnostic as to the cause of failure in TMR sensors.

At decision block 260, the TMR diagnostic program 101 tests whether theratio is greater than a limit. More generally, in the depictedembodiment, responsive to the ratio being greater than a predeterminedthreshold (decision block 260, YES branch), the TMR diagnostic program101 determines, at step 280, that the short is caused by a magneticshunt, for example by a tape scratch and/or lapping scratch. Anexemplary value for the predetermined ratio is 1.7, wherein theinventors have observed and/or recognized that the method is diagnosticfor some TMR sensors, in at least one embodiment of the invention. Inalternative embodiments, functional values for the predetermined ratiofor other TMR configurations, for example having different geometries,can be determined by engineers without undue experimentation, accordingto the models described herein. In addition to the ratio, the TMRdiagnostic program 101 may require that the change in resistance and thechange in voltage amplitude both be negative. By contrast, in thedepicted embodiment, responsive to the ratio being less than thepredetermined ratio threshold (decision block 260, NO branch), or,additionally, responsive to the either the change in resistance or thechange in voltage amplitude being positive, the TMR diagnostic program101 determines at step 285 that the short is caused by at least one of adielectric breakdown and a nonmagnetic shunt.

Stated differently, in the embodiment depicted in in FIG. 2, for a TMRsensor used in a tape drive to read magnetic data from the tape, measurethe change in the resistance, ΔR_(TMR), and the change in the amplitude,ΔAmp, of a TMR sensor used in a tape drive from their initial values,and take the ratio,

$\frac{\Delta \; {Amp}}{\Delta \; R_{TMR}},$

and if ΔAmp and ΔR_(TMR) are both negative and their ratio is largerthan a given value, dAdR_(Limit), then the shorting mechanism is amagnetic shunt from a scratch, and if the ratio is less thandAdR_(Limit), then the short is due to dielectric breakdown or anon-magnetic shunt, where an exemplary value of dAdr_(Limit) is 1.7.

Referring still to the embodiment depicted in FIG. 2, responsive todetermining that the short is caused by a dielectric breakdown (step285), the TMR diagnostic program 101, at step 295, limits a bias voltageacross the TMR sensor to no greater than a voltage limit. The voltagelimit is set at a value that is effective for protecting the dielectricbreakdown from growing. An exemplary voltage limit for variouscontemplated embodiments is between 175 mV and 200 mV.

Referring still to the embodiment depicted in FIG. 2, responsive todetermining that said short is caused by a magnetic shunt (step 280),the TMR diagnostic program 101, at step 290, sets a bias voltage acrossthe TMR sensor to a normal value, which is effective in the absence ofthe short. An exemplary normal value for contemplated embodiments isbetween 200 mV and 300 mV.

Stated differently, if the cause of the drop in amplitude and resistanceis determined to be due to a magnetic shunt, then the bias voltageacross the TMR tunnel junction may be set to a value as would have beendeemed safe (for example, by one of skill in the art for a particularembodiment) for a TMR tunnel junction prior to the development of themagnetic shunt. Similarly, if the cause of the drop in amplitude andresistance is determined to be due to a dielectric breakdown, then thebias voltage across the TMR tunnel junction may be set to a value whichis lower than was previously deemed safe for a TMR sensor which had notundergone dielectric breakdown, and which is safe for a TMR sensor ofthe area and thickness of the TMR that has undergone dielectricbreakdown.

In alternative embodiments, the TMR diagnostic program 101 measures thechange in voltage amplitude as per step 220 over a range of values ofthe change in resistance. Correspondingly, at step 240, the TMRdiagnostic program 101 determines the ratio over the range. Thus, insuch embodiments, responsive to both: (i) the ratio is greater than thepredetermined ratio threshold where the change in resistance than apredetermined drop threshold; and (ii) the ratio is less than thepredetermined ratio threshold where the change in resistance is morenegative than the predetermined resistance drop threshold, the TMRdiagnostic program 101 determines that the short is caused by themagnetic shunt. An exemplary value for the predetermined drop thresholdis −25%. In physical terms, if the voltage amplitude fails to continueto drop as fast as resistance across the TMR sensor, then it is possibleto conclude that the shunt is not getting worse, and is therefore amagnetic shunt, and not a dielectric breakdown or nonmagnetic shunt.

Referring now to FIG. 3, FIG. 3 is a flowchart diagram of a TMRdiagnostic program 101, in accordance with at least one embodiment ofthe present invention. For the depicted embodiment, the method may beunderstood to be performed by one or more processors in electroniccommunication with a TMR sensor. In one embodiment, the TMR sensor isinstalled in the read head of a magnetic tape drive. In anotherembodiment, the TMR sensor is installed in the read head of a hard diskdrive. More generally, the TMR sensor may be a component of a magneticstorage drive configured to read magnetic data from a magnetic storagemedium. In the depicted embodiment, at step 300, the TMR diagnosticprogram 101 detects an operational anomaly. An operational anomaly mayinclude a general drop in resistance (according to any type ofmeasurement) across the TMR sensor, as compared with an earlier time forthe same TMR sensor or a TMR sensor of similar geometry. At a higherlevel, an operational anomaly may be detected in read errors or reducedperformance of the reading device of which the TMR sensor is acomponent. For example, in a tape or hard disk drive, automated errorcorrection may require multiple rereadings of the same data due to theoperational anomaly, which can manifest in repetitive drive activity anddelayed access times to data on disk or tape.

With reference to FIGS. 12A and 12B, the TMR diagnostic program 101 may,responsive to detecting the operational anomaly, detect a short usingpositive and negative bias currents. In a TMR sensor undamaged by ashort, as per FIG. 12A, resistance is expected to decrease in thepresence of a bias current with either polarity, however each polaritymay behave differently with resistance dropping in differing amountsdepending on the polarity of the bias current. By contrast, FIG. 12Bshows resistance increasing with bias current in the presence of a shortby joule heating, according to the principle that resistance in ordinaryconductive materials increases with temperature. FIG. 12B shows anextreme case where an actual increase in resistance is observed withbias current, however more generally, a change in resistance that, evenif negative, is more positive than expected is diagnostic of a short.The expected change in resistance can be modeled for a given devicegeometry. In the context of an implemented device in a tape drive, theexpected device properties, including the expected resistance changeunder bias current of varying magnitude and polarity, can be known atdesign time and made available as parameters to the TMR diagnosticprogram 101.

More particularly, the TMR diagnostic program 101 may diagnose a shortusing bias currents by applying a first positive bias current across theTMR sensor, measuring a first positive resistance in the presence of thefirst positive bias current, applying a second positive bias currentacross the TMR sensor, measuring a second positive resistance change inthe presence of the second positive bias current, determining a positiveresistance change based on the first positive bias current and thesecond positive bias current (e.g., by computing the difference betweenthem), applying a first negative bias current across the TMR sensor,measuring a first negative resistance change in the presence of saidfirst negative bias current, applying a second negative bias currentacross the TMR sensor, measuring a second negative resistance in thepresence of the second negative bias current, determining a negativeresistance change based on the first negative bias current and thesecond negative bias current (e.g., by computing the difference betweenthem), and, responsive to at least one of the positive resistance changeand the negative resistance change being more positive than apredetermined short detection limit, determining that a short has beendetected.

The predetermined short detection limit may be set based on the deviceproperties and the desired conservativeness, according to engineeringconsiderations. The first positive bias current and the second negativebias current may be set at a sufficiently low magnitude to effectivelynot affect measured resistance for said tunneling magnetoresistivesensor. The second positive bias current and second negative biascurrent may be set at a sufficiently greater magnitude than said firstpositive bias current to affect measured resistance for said tunnelingmagnetoresistive sensor. Still more particularly, the second positivebias current and second negative bias current may be of a magnitude thatis less than an operational limit based on the device geometry and atleast sufficiently great to consistently affect measured resistance forsaid tunneling magnetoresistive sensor.

At step 310, the TMR diagnostic program 101 applies a positive biascurrent across the TMR sensor and measures a first resistance change inthe presence of the positive bias current. At step 320, the TMRdiagnostic program 101 applies a negative bias current across the TMRsensor and measures a second resistance change in the presence of thenegative bias current. In the context of the embodiment depicted in FIG.3, “positive” and “negative” currents may be understood as currents inopposing directions, with an arbitrary selection of the direction inwhich the currents are opposed. Similarly, the invention may be appliedto any two opposing directions of current regardless of whether currentis modeled as a flow of negative electrons or as an opposing flow ofpositive current. Additionally, a resistance change may be generallyunderstood as the difference between a pre-short measurement of theresistance of the TMR sensor and a measurement in the presence of theshort and/or one or more other conditions.

Referring still to the embodiment depicted in FIG. 3, at decision block330, the TMR diagnostic program 101 determines the sign of the change inresistance for the first and second changes in resistance. The sign ofthe change in resistance may be understood such that a positive changein resistance denotes an increase in resistance. The underlying physicalmechanism for some shorts is that, where a thin electrically conductivebridge exists across the tunnel junction layer, applying a bias currentin at least one direction will cause the thin bridge to heat up, whichincreases resistance. In the embodiment depicted in FIG. 3, at decisionbock 340 (Decision block 330, YES branch), responsive to either thefirst resistance change being relatively positive or the secondresistance change being relatively positive, at step 370 (decision block340, NO branch) the TMR diagnostic program 101 returns a probabledetermination that the operational anomaly is caused by a short, such asan electrically conductive bridge (e.g., a pin-hole dielectricbreakdown) across the tunnel junction layer of the tunnelingmagnetoresistive sensor.

Referring still to the embodiment depicted in FIG. 3, at decision block340 (decision block 330, YES branch), the TMR diagnostic program 101determines whether both of the first resistance change and the secondresistance change are positive, which denotes a resistance increase forboth polarities across the TMR sensor. Responsive to both the firstresistance change being positive and the second resistance change beingpositive, at step 365, returning a definite determination that saidshort is caused by the electrically conductive bridge. As used herein, aprobable determination may be understood generally as being less certainthan a definite determination, and a definite determination may beunderstood as arbitrarily certain up to effectively absolute certainty.Thus, a probable determination implies relatively conservativeengineering assumptions (whatever those may be for a particularembodiment) regarding protecting the TMR from further dielectricbreakdown as compared with a definite determination.

Referring still to the embodiment of FIG. 3, stated differently, if theresistance of a TMR sensor's tunnel junction increases with bias currentfor either positive or negative current, then the TMR sensor most likelyhas a short. If the resistance of a TMR sensor's tunnel junctionincreases with bias current for both positive and negative current, thenthe TMR sensor conclusively has an electrically conductive short.

Referring still to the embodiment depicted in FIG. 3, at decision block350 (decision block 340, NO branch), the TMR diagnostic program 101tests whether the magnitude of the change in a measured value R_(COLD)is greater than zero (where magnitude is expressed using the absolutevalue pipe symbol, given as |ΔR_(COLD)|). R_(COLD) may be understood asthe measured resistance in the near absence of bias current. Morespecifically, due to the small scale of the TMR sensor, it is notpossible to measure R with a truly neutral bias current because electricfields and currents are inherent at that scale. Thus, R_(COLD) ismeasured when the effective bias current in the desired direction acrossthe TMR sensor is brought as close to zero or neutral as possible. Toachieve the change in R_(COLD), the TMR diagnostic program 101 comparescurrently measured R_(COLD) with a previously measured or expectedR_(COLD) for the TMR sensor prior to the operational anomaly. In caseswhere the previously measured value for the same device is not known,for example in a newly operational TMR sensor, R_(COLD) may be comparedwith a measured value for a similar TMR sensor of similar geometry andcomposition. In the contemplated embodiment, the TMR diagnostic program101 computes |ΔR_(COLD)| by taking the magnitude or absolute value(i.e., regardless of whether absolute resistance has increased ordecreased) of R_(COLD). The inventors have observed and/or recognizedthat a change in absolute resistance measured in the effective absenceof bias current without a corresponding ΔR₊ or Δ_(R−) (i.e.,|ΔR_(COLD)|>ΔR_(COLD) _(LIMIT) , where ΔR_(COLD) _(LIMIT) is a near-zeropositive predetermined limit value sufficient to distinguish effectivelyzero resistance change), is diagnostic as to a change in magnetic stateof the TMR sensor as a whole device (step 360), which the inventors haveobserved to occur with some probability in TMR sensors. As a cause of aresistance drop, a change in magnetic state is understood by theinventors to be distinct from a short, however caused, and thus notnecessarily requiring corrective action.

Referring now to FIG. 4, FIG. 4 is a flowchart diagram for a TMRdiagnostic program 101, in accordance with at least one embodiment ofthe present invention. For the depicted embodiment, the method isunderstood to be performed by one or more processors in electroniccommunication with a TMR sensor, wherein said tunneling magnetoresistivesensor is a component of a magnetic tape drive or hard disk configuredto read magnetic data from a magnetic tape or hard disk. Diagnostically,the properties and inferences applied in a TMR diagnostic program 101according to FIG. 4 rely upon the assumption of a physical model of amagnetic medium, specifically the tape or disk, running across thesensor in conjunction with detecting the below-described electricalproperties. In the depicted embodiment, at step 400, the TMR diagnosticprogram 101 detects a short contemporaneously with the tape (or harddisk or other magnetic storage medium) running across the TMR sensor. Itshould be noted that contemporaneous operation permits a measurement ofvoltage amplitude across the TMR sensor, because the movement of thetape or other medium is what causes resistance, and thus measuredvoltage, to vary with time. By contrast, only stopping the motion of thetape or other medium permits static resistance of the TMR to bemeasured, because motion of the medium causes resistance to vary. Asdescribed above, the short is detected by observing a drop in resistanceacross the TMR sensor. At step 410, the TMR diagnostic program measuresa voltage amplitude across the TMR sensor as a function of a fractionalcurrent through said tunneling magnetoresistive sensor to yield avoltage amplitude data set. At step 420, the TMR diagnostic program 101fits the voltage amplitude data set to a power law using any functionmatching and/or regression technique.

If the resistance of a TMR sensor drops while running against tape, theinventors have observed and/or recognized that the short can be ascribedto a magnetic shunt shorting across the tunnel junction if theamplitude, Amp, versus fractional current through the TMR sensor, givenby Equation 8, above, can be fit with the power law equation:

$\begin{matrix}{{Amp}_{0} = \left( \frac{I_{TMR}}{I_{0}} \right)^{\beta}} & {{Equation}\mspace{14mu} 19}\end{matrix}$

Where β is greater than 2.5, and where I₀ is a fixed bias current.R isthe resistance with the short, and R_(TMRo) is the TMR resistance priorto the short and Amp_(o) is the amplitude measured with the bias currentI_(o) prior to the short. Also, thus, fractional current may be definedas a bias current multiplied by a measured resistance after the shortand divided by an initial resistance prior to the short. Also, thus, thepower law includes an initial voltage amplitude prior to the shortmultiplied by a ratio raised to the exponent. The ratio includes thefractional current divided by said bias current.

Referring still to the embodiment of FIG. 4, the TMR diagnostic program101, at decision block 430, tests whether the exponent exceeds athreshold. As above, an exemplary range of the exponent threshold isbetween 2.0 and 2.5. At step 440, in the depicted embodiment, the TMRdiagnostic program, responsive to the voltage amplitude data set fittinga power law, wherein the power law comprises an exponent, and whereinthe exponent is greater than the exponent threshold, determines that theshort is caused by a magnetic shunt. At step 450, the TMR diagnosticprogram set the bias current to a normal value, the normal value beingeffective in the absence of the short. As above, exemplary values of anormal value are between.

By contrast, where the exponent does not exceed the threshold, or wherethere is no power law fit, then, at step 445 (decision block 430, NObranch), the TMR diagnostic program 101 may conclude that the short isdue to dielectric breakdown or a nonmagnetic shunt. Accordingly, at step445, the TMR diagnostic program 101 may limit the bias voltage on theTMR sensor to a safe level, as described more particularly above.

In another embodiment of the invention, for TMR sensors installed in aread head of a tape drive, hard disk drive, or similar device, a TMRdiagnostic program 101 measures the read-back amplitude, Amp_(m) (i.e.,voltage amplitude) and the resistance, R_(m), for track m (wherein theread had includes multiple parallel tracks for reading from the tape,disk, or other magnetic storage medium). In such an embodiment, a TMRdiagnostic program 101 measures the average or median voltage amplitude,

Amp

, and resistance,

R

for neighbor or all tracks in the read head. If for a given track, m,(Amp_(m)−

(Amp

)<dAmp_(Error). That is, where dAmp_(Error), is a predetermined value,and the ratio

$\frac{\; {{Amp}_{m} - {\langle{Amp}\rangle}}}{R_{m} - {\langle\; R\rangle}}$

is greater than a predetermined value, dAmpdR_(Limit), then the TMRdiagnostic program determines that track m has suffered a short from amagnetic shunt.

FIG. 18 is a block diagram depicting components of a computer 1800suitable for executing the TMR diagnostic program 101. FIG. 18 displaysthe computer 1800, the one or more processor(s) 1804 (including one ormore computer processors), the communications fabric 1802, the memory1806, the RAM, the cache 1816, the persistent storage 1808, thecommunications unit 1810, the I/O interfaces 1812, the display 1820, andthe external devices 1818. It should be appreciated that FIG. 18provides only an illustration of one embodiment and does not imply anylimitations with regard to the environments in which differentembodiments may be implemented. Many modifications to the depictedenvironment may be made.

As depicted, the computer 1800 operates over a communications fabric1802, which provides communications between the cache 1816, the computerprocessor(s) 1804, the memory 1806, the persistent storage 1808, thecommunications unit 1810, and the input/output (I/O) interface(s) 1812.The communications fabric 1802 may be implemented with any architecturesuitable for passing data and/or control information between theprocessors 1804 (e.g., microprocessors, communications processors, andnetwork processors, etc.), the memory 1806, the external devices 1818,and any other hardware components within a system. For example, thecommunications fabric 1802 may be implemented with one or more buses ora crossbar switch.

The memory 1806 and persistent storage 1808 are computer readablestorage media. In the depicted embodiment, the memory 1806 includes arandom access memory (RAM). In general, the memory 1806 may include anysuitable volatile or non-volatile implementations of one or morecomputer readable storage media. The cache 1816 is a fast memory thatenhances the performance of computer processor(s) 1804 by holdingrecently accessed data, and data near accessed data, from memory 1806.

Program instructions for the TMR diagnostic program 101 may be stored inthe persistent storage 1808 or in memory 1806, or more generally, anycomputer readable storage media, for execution by one or more of therespective computer processors 1804 via the cache 1816. The persistentstorage 1808 may include a magnetic hard disk drive. Alternatively, orin addition to a magnetic hard disk drive, the persistent storage 1808may include, a solid state hard disk drive, a semiconductor storagedevice, read-only memory (ROM), electronically erasable programmableread-only memory (EEPROM), flash memory, or any other computer readablestorage media that is capable of storing program instructions or digitalinformation.

The media used by the persistent storage 1808 may also be removable. Forexample, a removable hard drive may be used for persistent storage 1808.Other examples include optical and magnetic disks, thumb drives, andsmart cards that are inserted into a drive for transfer onto anothercomputer readable storage medium that is also part of the persistentstorage 1808.

The communications unit 1810, in these examples, provides forcommunications with other data processing systems or devices. In theseexamples, the communications unit 1810 may include one or more networkinterface cards. The communications unit 1810 may provide communicationsthrough the use of either or both physical and wireless communicationslinks. TMR diagnostic program 101 may be downloaded to the persistentstorage 1808 through the communications unit 1810. In the context ofsome embodiments of the present invention, the source of the variousinput data may be physically remote to the computer 1800 such that theinput data may be received and the output similarly transmitted via thecommunications unit 1810.

The I/O interface(s) 1812 allows for input and output of data with otherdevices that may operate in conjunction with the computer 1800. Forexample, the I/O interface 1812 may provide a connection to the externaldevices 1818, which may include a keyboard, keypad, a touch screen,and/or some other suitable input devices. External devices 1818 may alsoinclude portable computer readable storage media, for example, thumbdrives, portable optical or magnetic disks, and memory cards. Softwareand data used to practice embodiments of the present invention may bestored on such portable computer readable storage media and may beloaded onto the persistent storage 1808 via the I/O interface(s) 1812.The I/O interface(s) 1812 may similarly connect to a display 1820. Thedisplay 1820 provides a mechanism to display data to a user and may be,for example, a computer monitor.

The programs described herein are identified based upon the applicationfor which they are implemented in a specific embodiment of theinvention. However, it should be appreciated that any particular programnomenclature herein is used merely for convenience, and thus theinvention should not be limited to use solely in any specificapplication identified and/or implied by such nomenclature.

The present invention may be a system, a method, and/or a computerprogram product at any possible technical detail level of integration.The computer program product may include a computer readable storagemedium (or media) having computer readable program instructions thereonfor causing a processor to carry out aspects of the present invention.

The computer readable storage medium can be a tangible device that canretain and store instructions for use by an instruction executiondevice. The computer readable storage medium may be, for example, but isnot limited to, an electronic storage device, a magnetic storage device,an optical storage device, an electromagnetic storage device, asemiconductor storage device, or any suitable combination of theforegoing. A non-exhaustive list of more specific examples of thecomputer readable storage medium includes the following: a portablecomputer diskette, a hard disk, a random access memory (RAM), aread-only memory (ROM), an erasable programmable read-only memory (EPROMor Flash memory), a static random access memory (SRAM), a portablecompact disc read-only memory (CD-ROM), a digital versatile disk (DVD),a memory stick, a floppy disk, a mechanically encoded device such aspunch-cards or raised structures in a groove having instructionsrecorded thereon, and any suitable combination of the foregoing. Acomputer readable storage medium, as used herein, is not to be construedas being transitory signals per se, such as radio waves or other freelypropagating electromagnetic waves, electromagnetic waves propagatingthrough a waveguide or other transmission media (e.g., light pulsespassing through a fiber-optic cable), or electrical signals transmittedthrough a wire.

Computer readable program instructions described herein can bedownloaded to respective computing/processing devices from a computerreadable storage medium or to an external computer or external storagedevice via a network, for example, the Internet, a local area network, awide area network and/or a wireless network. The network may comprisecopper transmission cables, optical transmission fibers, wirelesstransmission, routers, firewalls, switches, gateway computers and/oredge servers. A network adapter card or network interface in eachcomputing/processing device receives computer readable programinstructions from the network and forwards the computer readable programinstructions for storage in a computer readable storage medium withinthe respective computing/processing device.

Computer readable program instructions for carrying out operations ofthe present invention may be assembler instructions,instruction-set-architecture (ISA) instructions, machine instructions,machine dependent instructions, microcode, firmware instructions,state-setting data, configuration data for integrated circuitry, oreither source code or object code written in any combination of one ormore programming languages, including an object oriented programminglanguage such as Smalltalk, C++, or the like, and procedural programminglanguages, such as the “C” programming language or similar programminglanguages. The computer readable program instructions may executeentirely on the user's computer, partly on the user's computer, as astand-alone software package, partly on the user's computer and partlyon a remote computer or entirely on the remote computer or server. Inthe latter scenario, the remote computer may be connected to the user'scomputer through any type of network, including a local area network(LAN) or a wide area network (WAN), or the connection may be made to anexternal computer (for example, through the Internet using an InternetService Provider). In some embodiments, electronic circuitry including,for example, programmable logic circuitry, field-programmable gatearrays (FPGA), or programmable logic arrays (PLA) may execute thecomputer readable program instructions by utilizing state information ofthe computer readable program instructions to personalize the electroniccircuitry, in order to perform aspects of the present invention.

Aspects of the present invention are described herein with reference toflowchart illustrations and/or block diagrams of methods, apparatus(systems), and computer program products according to embodiments of theinvention. It will be understood that each block of the flowchartillustrations and/or block diagrams, and combinations of blocks in theflowchart illustrations and/or block diagrams, can be implemented bycomputer readable program instructions.

These computer readable program instructions may be provided to aprocessor of a general purpose computer, special purpose computer, orother programmable data processing apparatus to produce a machine, suchthat the instructions, which execute via the processor of the computeror other programmable data processing apparatus, create means forimplementing the functions/acts specified in the flowchart and/or blockdiagram block or blocks. These computer readable program instructionsmay also be stored in a computer readable storage medium that can directa computer, a programmable data processing apparatus, and/or otherdevices to function in a particular manner, such that the computerreadable storage medium having instructions stored therein comprises anarticle of manufacture including instructions which implement aspects ofthe function/act specified in the flowchart and/or block diagram blockor blocks.

The computer readable program instructions may also be loaded onto acomputer, other programmable data processing apparatus, or other deviceto cause a series of operational steps to be performed on the computer,other programmable apparatus or other device to produce a computerimplemented process, such that the instructions which execute on thecomputer, other programmable apparatus, or other device implement thefunctions/acts specified in the flowchart and/or block diagram block orblocks.

The flowchart and block diagrams in the Figures illustrate thearchitecture, functionality, and operation of possible implementationsof systems, methods, and computer program products according to variousembodiments of the present invention. In this regard, each block in theflowchart or block diagrams may represent a module, segment, or portionof instructions, which comprises one or more executable instructions forimplementing the specified logical function(s). In some alternativeimplementations, the functions noted in the blocks may occur out of theorder noted in the Figures. For example, two blocks shown in successionmay, in fact, be executed substantially concurrently, or the blocks maysometimes be executed in the reverse order, depending upon thefunctionality involved. It will also be noted that each block of theblock diagrams and/or flowchart illustration, and combinations of blocksin the block diagrams and/or flowchart illustration, can be implementedby special purpose hardware-based systems that perform the specifiedfunctions or acts or carry out combinations of special purpose hardwareand computer instructions.

What is claimed is:
 1. A computer-implemented method comprising:detecting a short across a tunneling magnetoresistive sensor; measuringa change in resistance of the tunneling magnetoresistive sensor;measuring a change in voltage amplitude for the tunnelingmagnetoresistive sensor; dividing the change in voltage amplitude by thechange in resistance to yield a ratio; and responsive to the ratio beinggreater than a predetermined ratio threshold, determining that the shortis caused by a magnetic shunt.
 2. The method of claim 1, wherein thetunneling magnetoresistive sensor is a component of a magnetic storagedrive configured to read magnetic data from a magnetic storage medium.3. The method of claim 1, wherein the predetermined ratio threshold isabout 1.7.
 4. The method of claim 1, wherein the change in voltageamplitude is measured over a range for the change in resistance, themethod further comprising: determining the ratio over the range; anddetermining that the short is caused by the magnetic shunt in responseto one of: the ratio being greater than the predetermined ratiothreshold where the change in resistance is more positive than apredetermined resistance drop threshold, or the ratio being less thanthe predetermined ratio threshold where the change in resistance is morenegative than the predetermined resistance drop threshold.
 5. The methodof claim 4, wherein the predetermined resistance drop threshold is about−25%.
 6. The method of claim 1, further comprising: responsive to theratio being less than the predetermined ratio threshold, determiningthat the short is caused by at least one of a dielectric breakdown or anonmagnetic shunt.
 7. The method of claim 6, further comprising:responsive to determining that the short is caused by the dielectricbreakdown, limiting a bias voltage across the tunneling magnetoresistivesensor to no greater than a voltage limit, the voltage limit being avalue effective for protecting the dielectric breakdown from growing. 8.The method of claim 1, further comprising: responsive to determiningthat the short is caused by the magnetic shunt, setting a bias voltageacross the tunneling magnetoresistive sensor to a normal value, thenormal value being effective in the absence of the short.
 9. A systemcomprising: a memory; and a processor communicatively coupled to thememory, wherein the processor is configured to perform a methodcomprising: detecting a short across a tunneling magnetoresistivesensor; measuring a change in resistance of the tunnelingmagnetoresistive sensor; measuring a change in voltage amplitude for thetunneling magnetoresistive sensor; dividing the change in voltageamplitude by the change in resistance to yield a ratio; and responsiveto the ratio being greater than a predetermined ratio threshold,determining that the short is caused by a magnetic shunt.
 10. The systemof claim 9, wherein the system further comprises a magnetic storagedrive configured to read magnetic data from a magnetic storage medium,wherein the tunneling magnetoresistive sensor is a component of themagnetic storage drive.
 11. The system of claim 9, wherein the change involtage amplitude is measured over a range for the change in resistance,wherein the method further comprises: determining the ratio over therange; and determining that the short is caused by the magnetic shunt inresponse to one of: the ratio being greater than the predetermined ratiothreshold where the change in resistance is more positive than apredetermined resistance drop threshold, or the ratio being less thanthe predetermined ratio threshold where the change in resistance is morenegative than the predetermined resistance drop threshold.
 12. Thesystem of claim 9, wherein the method further comprises: responsive tothe ratio being less than the predetermined ratio threshold, determiningthat the short is caused by at least one of a dielectric breakdown or anonmagnetic shunt.
 13. The system of claim 12, wherein the methodfurther comprises: responsive to determining that the short is caused bythe dielectric breakdown, limiting a bias voltage across the tunnelingmagnetoresistive sensor to no greater than a voltage limit, the voltagelimit being a value effective for protecting the dielectric breakdownfrom growing.
 14. The system of claim 9, wherein the method furthercomprises: responsive to determining that the short is caused by themagnetic shunt, setting a bias voltage across the tunnelingmagnetoresistive sensor to a normal value, the normal value beingeffective in the absence of the short.
 15. A computer program productcomprising a computer readable storage medium having programinstructions embodied therewith, the program instructions executable byprocessor to cause the processor to perform a method comprising:detecting a short across a tunneling magnetoresistive sensor; measuringa change in resistance of the tunneling magnetoresistive sensor;measuring a change in voltage amplitude for the tunnelingmagnetoresistive sensor; dividing the change in voltage amplitude by thechange in resistance to yield a ratio; and responsive to the ratio beinggreater than a predetermined ratio threshold, determining that the shortis caused by a magnetic shunt.
 16. The computer program product of claim15, wherein the predetermined ratio threshold is about 1.7.
 17. Thecomputer program product of claim 15, wherein the change in voltageamplitude is measured over a range for the change in resistance, whereinthe method further comprises: determining the ratio over the range; anddetermining that the short is caused by the magnetic shunt in responseto one of: the ratio being greater than the predetermined ratiothreshold where the change in resistance is more positive than apredetermined resistance drop threshold, or the ratio being less thanthe predetermined ratio threshold where the change in resistance is morenegative than the predetermined resistance drop threshold.
 18. Thecomputer program product of claim 15, wherein the method furthercomprises: responsive to the ratio being less than the predeterminedratio threshold, determining that the short is caused by at least one ofa dielectric breakdown or a nonmagnetic shunt.
 19. The computer programproduct of claim 18, wherein the method further comprises: responsive todetermining that the short is caused by the dielectric breakdown,limiting a bias voltage across the tunneling magnetoresistive sensor tono greater than a voltage limit, the voltage limit being a valueeffective for protecting the dielectric breakdown from growing.
 20. Thecomputer program product of claim 15, wherein the method furthercomprises: responsive to determining that the short is caused by themagnetic shunt, setting a bias voltage across the tunnelingmagnetoresistive sensor to a normal value, the normal value beingeffective in the absence of the short.